The gemologist Tingting Gu, of the Gemological Institute of America (GIA) in New York (United States), was appraising and verifying a diamond, about to be embedded as another piece of jewelry in a ring, when she noticed under the microscope its geological significance.
It was the second ringwoodite diamond ever discovered.
To identify his find, Gu contacted Professor Fabrizio Nestola, a researcher at the Department of Geoscience at the University of Padua (Italy).
The rarity of the IaB diamond, as it is called in the study published in the journal
Nature
,
is that it is composed of the minerals ferropericlase, ringwoodite and enstatite.
"It is the first time that this combination has occurred, which confirms our experiments in the laboratory and gives us unprecedented knowledge about the composition and structure of one of the most inaccessible and remote places on Earth", enthuses Nestola, co-author of the study. international.
The 1.5-centimeter gem, and in good condition, came from the Karowe mine in Botswana (Africa) and its analysis suggests that 660 kilometers underground there are minerals in contact with water.
The work changes the known history of the Earth's subsoil, since water is much more present in its geochemical composition than was intuited.
The discovery gives us unprecedented knowledge about the composition and structure of one of the most inaccessible and remote places on Earth
Fabrizio Nestola, researcher at the Department of Geoscience at the University of Padua (Italy)
Diamonds are (geological) time machines.
They were formed in the depths of the earth millions of years ago, based on pressure and high temperatures, to later be expelled due to the subduction of tectonic plates through volcanoes and earthquakes.
These minerals are one of the best ways to find out what happens thousands of kilometers deep inside the Earth, an environment to which scientists do not have direct access.
The linchpin of the diamond in this latest study is ringwoodite, a magnesium silicate mineral first discovered on Earth in 1969 in a meteorite that struck Australia.
Subsequently, the first terrestrial sample of this mineral was excavated in 2014, sealed in a “super-deep diamond”, underlines Nestola, at the Juína mine in Brazil.
The finding confirmed the theories about what the subsoil could be like at that depth, between 400 and 600 kilometers, which until now has only been possible to analyze via sediments expelled by geological cataclysms.
It probably emerged from the depths embedded in volcanic rock called a kimberlite chimney, millions of years ago.
“This was of great help”, notes Nestola, “because the largest well that humanity has built
only
It is 12 kilometers long.
Detailed plan of the diamond, where the analysis highlights a composition of ferropericlase (bluish center), ringwoodite (upper edge) and enstatite (lower edge). Nathan D. Renfro and Tingting Gu (GIA)
Ringwoodite is nothing more than an olivine, one of the most common minerals in the upper mantle of the Earth, just below the crust, "to which great atmospheric pressure has been applied", specifies the geologist Javier García Guinea, from the Museum National Natural Sciences of the CSIC.
For him, who is not linked to this research, the work is "continuous", but he understands that "this is science, and it is done step by step".
The analysis of IaB by Gu's team places it in the transition zone between the second and third layers of the Earth, at a depth of about 400 and 670 kilometers, where this diamond was formed, at a pressure of 23.5 GPa (gigapascals) and about 1,650 Cº.
As a reference, Nestola ironically compares: "The pressure that crushes the atoms of the mineral until it turns into diamond is immense, a single gigapascal is equivalent to four Mount Everests above your head."
The presence of H₂O in the lower mantle has consequences for the structure and evolution of the planet
Antonio García Casco, geologist of the Department of Mineralogy and Petrology of the University of Granada
The chemical composition of the diamond suggests the presence of the equivalent of oceans of water between the underground layers, "something that is not new and has been known for decades," explains Antonio García Casco, a geologist from the Department of Mineralogy and Petrology of the University from Granada.
But kilometers deep, it is not liquid water as we understand it on the surface.
The H₂O "becomes a fluid, half liquid, half gas" and adheres to minerals that may contain up to 10% to 20% of it by weight", the professor develops.
Casco considers the published work to be important due to the possibility of “inferring the presence of free H₂O in the lower mantle”, which has consequences for “the structure and evolution of the planet;
for example, convection in the mantle and plate tectonics, which irremediably conditions us”.
For the researcher, this study is an opportunity for mineralogy to witness transformation processes that only occur "at depths to which we will never have direct access."
The diamond, saved
in extremis
from ending up in an engagement ring, “freezes and captures its environment, and then transports it, like a taxi, from the depths until it reaches the light”, synthesizes Nestola.
Moreover, for a geologist like him, the more material he absorbs, the more valuable it is to study.
"Just the opposite for a jeweler," laughs the co-author.
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